Step-by-step explanation:
Imagine lying out 2 sticks with lengths 9 and 12
on the ground. The maximum length you can
get is by putting them in a straight line. (9 + 12 = 21)
However, 21 is less than 24, which is
supposed to be the 3rd side of the triangle.
Hence the set 9, 12, 24 cannot form a triangle.
